[metacity] boxes: fix build warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit d44180053525b80c4da4d064c326cf0d418335cb
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sun Sep 20 04:42:09 2015 +0300

    boxes: fix build warning

 src/core/boxes.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/core/boxes.c b/src/core/boxes.c
index 8c569aa..47ebf75 100644
--- a/src/core/boxes.c
+++ b/src/core/boxes.c
@@ -1526,6 +1526,8 @@ add_edges (GList               *cur_edges,
           temp_edge->rect.y      += temp_edge->rect.height;
           temp_edge->rect.height  = 0;
           break;
+        default:
+          break;
         }
       temp_edge->edge_type = META_EDGE_SCREEN;
       cur_edges = g_list_prepend (cur_edges, temp_edge);